; they adapt
one or both elements for independent temporary existence, and for mutual
union. The contrivances for these purposes are sometimes wonderfully
complex, as with the spermatophores of the Cephalopoda. The male element
sometimes possesses attributes which, if observed in an independent animal,
would be put down to instinct guided by sense-organs, as when the {384}
spermatozoon of an insect finds its way into the minute micropyle of the
egg, or as when the antherozoids of certain algae swim by the aid of their
ciliae to the female plant, and force themselves into a minute orifice. In
these latter cases, however, we must believe that the male element has
acquired its powers, on the same principle with the larvae of animals,
namely by successive modifications developed at corresponding periods of
life: we can hardly avoid in these cases looking at the male element as a
sort of premature larva, which unites, or, like one of the lower algae,
conjugates, with the female element. What determines the aggregation of the
gemmules within the sexual organs we do not in the least know; nor do we
know why buds are formed in certain definite places, leading to the
symmetrical growth of trees and corals, nor why adventitious buds may be
formed almost anywhere, even on a petal, and frequently upon healed
wounds.[912] As soon as the gemmules have aggregated themselves,
development apparently commences, but in the case of buds is often
afterwards suspended, and in the case of the sexual elements soon ceases,
unless the elements of the opposite sexes combine; even after this has
occurred, the fertilised germ, as with seeds buried in the ground, may
remain during a lengthened period in a dormant state.

The antagonism which has long been observed,[913] though exceptions
occur,[914] between active growth and the power of sexual
reproduction--between the repair of injuries and gemmation--and with
plants, between rapid increase by buds, rhizomes, &c., and the production
of seed, is partly explained by the gemmules not existing in sufficient
numbers for both processes. {385} But this explanation hardly applies to
those plants which naturally produce a multitude of seeds, but which,
through a comparatively small increase in the number of the buds on their
rhizomes or offsets, yield few or no seed. As, however, we shall presently
see that buds probably include tissue which has already been to a certain
extent developed or differentiated,